Automotive vehicles and small vessels use D.C. electrical power sources for operation of lights and controls; the traditional power source for these applications once was a D.C. generator driven from the vehicle engine. More recently, with major improvements in rectifier technology, the D.C. generator has been replaced by the combination of a small alternator and a rectifier. The most practical and most widely used type of alternator employs a rotating field, using a field coil mounted in a core formed by two magnetic steel core members with interleaved finger-like pole pieces. For these magnetic core members, precision manufacture is essential.
Traditional processes that have been employed in the manufacture of magnetic rotor core members for alternators and like dynamoelectric machines include the cold forging (or cold extrusion) process, the cold forming stamping process, and the hot forging process. These manufacturing procedures have each incorporated methods and techniques that have been developed independently and separately for each. Though significant improvements and advances in all of these methods have been achieve during past years, each of the traditional processes nevertheless still presents drawbacks and disadvantages which have proved difficult or impossible to overcome. Accordingly, each of the traditional procedures still leaves much to be desired in terms of yield rate, productivity, equipment required, etc.
For instance, the cold forging or cold extrusion method requires a large scale, high capacity press that affords an extremely high processing force. This presents substantial problems with respect to operating life and productivity of the tooling employed in the press. The cold forming stamping process presents a distinct disadvantage with respect to excessive consumption of the material from which a preliminary core blank is punched and an undesirably low yield rate. Further, this process cannot create an integral hub section, as used in many rotor core members, so that a separate rotor core spacer or hub has to be manufactured by some other process.
The hot forging process is inherently a higher yield rate procedure that has the further advantage of requiring less processing force than cold forging. However, hot forging alone is inadequate in attaining high dimensional accuracy and also is poorly adapted to producing a shaft aperture in the hub of the rotor core member. Consequently, the basic hot forging process must be followed by a number of machining steps to achieve the required finished form with precision controlled dimensional tolerances.
A superior method of manufacturing magnetic rotor core members for dynamoelectric machines is desired in the inventor's earlier U.S. Pat. No. 4,558,511 issued Dec. 17, 1985; the method disclosed in that patent employs a combination of hot forging and cold forging operations that minimizes many of the disadvantages of traditional processes. In the process disclosed in that patent, a segment of steel bar stock is first hot forged to form a preliminary core blank having a general approximation of the external configuration desired for the rotor core member and is then coined to form a secondary core blank closer to the final required configuration, after which the secondary core blank is gradually air cooled. At this stage, the secondary core blank may be cold punched to form a shaft aperture through its hub and simultaneously cold compressed for further shaping; alternatively, the secondary core blank may be rough machined on numerous surfaces, the rough machining also forming a shaft aperture. Finally, one or more cold compression steps complete the finished rotor core member, with no requirement for close tolerance finish machining.
Although the method of the inventor's earlier U.S. Pat. No. 4,558,511 affords a marked improvement over previously known techniques for manufacturing magnetic rotor core members, further improvement to meet current demands for higher productivity, improved dimensional accuracy, and lower manufacturing costs are highly desirable. The present invention is intended to and does afford improvements on the inventor's prior method, affording higher yield rates, further reduction of manufacturing steps, and more precise dimensional control.